Various applications of electronic circuitry involve the use of integrated circuits (ICs). ICs, for example, facilitate the ability to incorporate a very large number of circuit elements into a very small area. ICs are particularly useful when active components, such as transistors and diodes, are needed to implement a particular design. Using today's semiconductor technology, for example, hundreds of millions and even billions of active devices may be incorporated into a single IC.
Unfortunately, many circuit applications also require the use of passive components, such as resistors, capacitors, and inductors. In some applications, the formation of passive components may be directly implemented within the semiconductor die itself. Other applications, however, preclude the use of semiconductor die implementations of passive components, since the reactive or resistive density of the silicon process cannot support the magnitude of reactance or resistance that is required.
Maximum capacitance densities, for example, of today's silicon processes are on the order of 10 fempto-Farads per square micrometer (fF/μ2) Thus, the implementation of a 10 nF capacitance, using actual transistors within the semiconductor die, would require 1 square millimeter (mm2) of semiconductor die area, which precludes their use in many applications due to the excessive semiconductor area requirement. Many electronic designs, therefore, require the use of discrete capacitive components that are external to the semiconductor die, so that larger capacitance values may be implemented without the need to utilize semiconductor die area.
One such electronic design, for example, involves the use of the so-called “clean-up” phase locked loop (PLL). Clean-up PLLs are used to filter phase jitter and phase noise from a reference input clock. As such, their loop bandwidths are required to be quite small, so that phase variations due to high frequency noise may be substantially removed. Accordingly, a large portion of the semiconductor die area is occupied by the loop filter capacitance, since an inverse relationship exists between the loop bandwidth and the capacitance values required by the loop filter to achieve the required loop bandwidth.
Thus, the circuit designer is left with the choice of using a large amount of semiconductor die area to implement the loop filter capacitance, or to use a discrete capacitive element, external to the semiconductor die, to implement the loop filter capacitance. Methods continue to be developed, therefore, that provide the circuit designer with alternative options of implementing capacitive components within the semiconductor die. Such methods should increase the capacitance generated while minimizing the amount of semiconductor die area used.